The field of the invention relates to an air/fuel control system of an internal combustion engine coupled to a catalytic converter. In particular, the invention relates to adjusting the air/fuel ratio measurement of a post-catalyst proportional exhaust gas oxygen sensor to correct for the water gas shift reaction that occurs in the catalytic converter of the vehicle""s exhaust system.
To minimize undesirable emissions from an automotive vehicle, it is known to control the air/fuel (A/F) ratio supplied to the vehicle engine within a relatively narrow range around the stoichiometric ratio. Various specific methods are known to control the engine air/fuel ratio within this so-called conversion window. Some of these methods include calculating desired air/fuel ratios at various times based upon feedback signals from exhaust gas oxygen (EGO) sensors positioned both upstream and downstream of the vehicle""s catalyst. An example of such an approach is disclosed in U.S. Pat. No. 5,115,639. Furthermore, systems that include both upstream and downstream EGO sensors commonly employ the output signals of those sensors to perform other diagnostic and control functions, such as estimating oxygen storage in the vehicle""s catalyst(s), estimating catalyst sulfation, estimating catalyst degradation, and the like.
To achieve optimal air/fuel control and accurate operation of the various other diagnostic and control functions described above, it is important that the EGO sensors provide accurate measurements. This is particularly true when the post-catalyst sensor is a proportional exhaust gas oxygen sensor, such as a conventional universal exhaust gas oxygen (UEGO) sensor, which provides an output signal indicative of the actual measured air/fuel ratio (as opposed to a two-state EGO sensor that only provides an indication that the air/fuel ratio is either rich or lean). However, the inventors hereof have recognized that a post-catalyst proportional UEGO sensor may systematically provide somewhat inaccurate measurements as a result of a water gas shift reaction that occurs inside of the vehicle""s exhaust stream between the pre-catalyst and post-catalyst sensors. Specifically, during steady-state operation (either rich or lean), the air/fuel ratio measured by the post-catalyst UEGO sensor should be substantially identical to the air/fuel ratio upstream of the catalyst. But the inventors hereof have recognized that the output of the post-catalyst UEGO sensor tends to indicate an air/fuel ratio that is less lean during lean operation of the engine and less rich during rich operation relative to the air/fuel ratio upstream of the catalyst. This is a systematic error that the inventors have attributed to the water gas shift reaction that occurs in the vehicle""s exhaust system. To achieve optimal air/fuel control and accurate results from the vehicle""s various diagnostic and control systems, it is desirable to adjust the output signal of the post-catalyst UEGO sensor to correct for the water gas shift reaction. Accordingly, the inventors have recognized that there is a need for a method and system to correct the output of a post-catalyst exhaust oxygen sensor.
The present invention comprises a method and system for adjusting an air/fuel ratio signal from a post-catalyst proportional UEGO sensor to correct for the water gas shift reaction that occurs in the vehicle""s exhaust stream between the pre-catalyst and post-catalyst sensors, particularly in the vehicle""s catalytic converter. The invention includes adjusting the output signal received from the post-catalyst UEGO sensor by adding a correction bias, which is derived by one of two preferred methods. According to the first preferred method, the correction bias is derived for a particular lean or rich air/fuel excursion during normal operation of the vehicle based upon actual output signals of the pre-catalyst and post-catalyst sensors received during a previous lean or rich excursion. Specifically, the correction bias is derived by taking the difference between the respective averages of the pre-catalyst and post-catalyst air/fuel ratios during a period of lean engine operation. Then, during a subsequent lean operation of the engine, the correction bias is added to the output signal of the post-catalyst UEGO sensor to adjust toward a leaner air/fuel ratio output. A similar method may be applied to adjust the output of the post-catalyst UEGO sensor during rich operation of the engine.
A second preferred method of the present invention includes pre-programming a correction bias function that provides correction biases as a function of the non-corrected output of the post-catalyst sensor. The pre-programmed function can take various forms, such as a formula, look-up table, or map profile, and is used to determine correction biases for the post-catalyst sensor at any time based upon the non-corrected output of the post-catalyst sensor. As in the first preferred method, the correction bias is added to the output signal of the post-catalyst sensor to adjust toward a leaner air/fuel ratio during lean operation and toward a richer air/fuel ratio during rich operation of the engine.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.